Method of coloring etched stainless steel



Patented ay 19, 1942 2,283,170 METHOD or comma ETCHED STAINLESS STEELClements Batcheller, Glens Falls, N. Y.

No Drawing. Application November 30, 1939,

Serial No. 306,974

11 Claims.

My invention relates to a new process-of which the following is aspecification whereby stainless steels which are cast, forged or rolledand have a decorative or utilitarian design etched therein may bechemically treated to produce color either in or about the etched zonesthereof and thus bring out the design in strong contrast to thesurrounding field.

In my copending application Ser. No. 301,805, filed October 28, 1939, Ihave disclosed a process whereby stainless steel products are firstchemically colored throughout, then offset printed in the desired designwith a suitable mask in the form of a resist, such as is commonly usedin the etching of metals, to protect certain of the colored areas whilethe color from the exposed areas is removed by an electrolytic strippingprocess to produce the design. This method works very satisfactorily onstainless steelsurfaces which have not been pre-etched to produce adesign thereon. However, I find that where certain portions of astainless steel surface have been pre-etched, a certain passivity isimparted to the metal in the etched zones which very sub stantiallyretards the coloration thereof when subjected to my coloration process.I propose to utilize this discovery in an entirely new process forproducing a desired design on a stainless steel surface.

The art of etching designs on metals as it has to prepare the specimensby printing either the field or the design thereon with an appropriateresist to protect the areas covered thereby from the action of theetching composition, and then to subject the printed metal to theetching solution. These processes require a subsequent filling in of theetched areas with color in order to accomplish the desired contrast.Such filling in is done either by spraying a colored lacquer onto theetched areas before the surrounding resist is removed from the unetchedareas, or is done after the removal of the resist by a trowelingoperation in which a color paste is packed into the etched areas. Ineither case a subsequent high temperature baking operation is necessaryto harden the color fill. However, even with the best known processesand the most skillful technique it has never been possible to producepermanent color fills in such etched products as are intended towithstand prolonged exposure to the Weather or to the destructive mediaencountered in many common industrial processes.

The principal object of my invention, therefore, is to provide animproved method of pro- 55 40 ployed. The steel is veying, astronomicaland drafting instruments,

clock faces and other dials, name plates, and other plates such as areordinarily attached to articles of manufacture and machines to indicatethe characteristics thereof, wiring diagrams, in-

15 struction charts, etc., are only a few of the many products which maybe produced by my process.

My process is only applicable to stainless steels which are iron alloyscontaining chromium, usually in quantities constituting 6% or more, by

weight, of the composition, and either with or without additions ofother elements such, for example, as aluminum, copper, manganese,molybdenum, nickel, silicon, titanium .and the like. By my processeither the etched portions may be colored and the field about them leftin the contrasting metal color of the steel, or the field about theetched zones may be colored and the etched zones left in the naturalcolor of the metal to produce the necessary contrast.

In accordance with my procedure, the steel surbeen commonly practicedfor many years is first face is first prepared for etching by printingthe design thereon with any of the resists, such as an asphaltcomposition commonly used in the etching of metals, which is capable ofprotecting the metal underneath from attack by the etching compositionemployed. One of the most common substances used for etching is ironperchloride, although solutionsof sulphuric acid, or mixtures of nitricand hydrofluoric acid are frequently emimmersed in the etching solutionuntil the desired depth of etch has been effected. So far as my processis concerned the depth of the etch is not of major importance and may beas little as 0.0015", because I primarily depend merely on thepassivation of the metal in the zones exposed to the etching solution.After the etching, the resist is removed by the application of asuitable solvent such as gasoline, benzol or the like.

Further treatment by my process, I find, is

facilitated if the metal is given a belt sanded finish known in thesteel industry as #4 finish. .Such a finish, which is effectedmechanically by specimen for rapid and uniform coloration of means of anabrasive belt, initially prepares my those areas surrounding the etch,and since the etched zones are lower than the surrounding zones they arenot affected by this treatment. It is to be understood that my coloringprocess may be applied to the steel without pre-sanding 5 are commonlyfurnished seem to passivate the surface to some degree and therebyretard the coloration.

The etched zones of the steel, however, are so passivated by the etchingsolution, and particularly if iron perchloride is employed, that itrequires an unusually long time to effect a coloration thereof by myprocess. In other words, when treated by my coloration process which Ishall hereinafter describe, the zones of the steel which are not etched,and particularly if these zones have been pro-treated by giving them abelt-sanded finish, as above described, can be colored a very deep black.while those zones which have been passivated by the etching solutionremain substantially in their natural etched color although all of thesurfaces are subjected to the same coloring treatment.

In my United States Patent Nos. 2,172,353 and 2,219,554 and in mycopendlng applications for Letters Patent Serial No. 206,043, filed May4,

1938, and Serial No. 241,990, filed November 23, 1938, I have describedprocesses for the coloration of stainless steels by immersion in asolution of concentrated sulphuric acid and water in which one or moreof a large number of etching inhibiting oxidizing agents are employed ina quantity sufficient to prevent etching of the steel during treatmentin the acid. By these processes a very beautiful deep black color may be1mparted to the steel.

The steel is colored by treating it, preferably by immersion in asolution of concentrated sulphuric-acid and water in which there isdissolved a sufiicient quantity of an appropriateetchinginhibiting-oxidizing agent to prevent etching of the surface ofthe steel during the coloring treatment. The acid solutions which Iemploy are comparatively highly concentrated and of such strength thatif used without an etching inhibitor would immediately attack andquickly dissolve 60 the steel. The solutions which I employ may containfrom about 23 to about 54 parts, by weight, of sulphuric acid (1.84 sp.g.) and from about 35 parts to about 64 parts, by weight, of

water. In other words, the solution of acid and 55 water may have anacid concentration (by weight) in round numbers from about 25% to about65%, but for the best results I prefer to use a solution having an acidconcentration, by

weight, varying from about 40% to about 55% of the acid-water solution.

In this acid-water solution I. dissolve a sufficient quantity of anappropriate etching-inhibiting-oxidizing agent to prevent etching of thesteel surface at the temperature at which the coloring process is to beconducted. Generally speaking, the quantity of etching inhibitornecessary will increase with the acid concentration and also with thetemperature, and it is possible to use such a'large proportion thereofas to prevent satisfactory coloration. In the wider ranges of acidconcentration given above, from about 4 to about 25 parts by weight ofinhibitor, constituting from about 3% to about 30% of the whole solutionmay be used, but I prefer to use the acid,

water and inhibitor in about the following proportions:

. Parts by weight Etching-inhibiting-oxidizing agent 10-14 Sulphuricacid (1.84 sp. g.) 36-50 Water 40 50.

ily determined for any acid concentration and any temperature by testingspecimens of the steel to be colored in the acid-water solution at thetemperature to be employed, and adding the inhibitor until attack on thesteel ceases.

As an inhibitor I prefer to use sodium or potassium dichromate but Ifind that any one or more of a large number of substances may beemployed. Included among the inhibitors are the chromates anddichromates of aluminum, ammonium, barium, bismuth, cadmium and calcium,cobalt, copper, iron, lead, lithium, magnesium, manganese, mercury,nickel, potassium, sodium, strontium and zinc; the vanadates ormetavanadates of ammonium, potassium or sodium, or vanadic acid ormetavanadic acid; chromium trioxide; the manganates or permanganates ofthe alkali metals of group I, the manganates or pennanganates of thealkaline earth metals of group II, and manganese dioxide.

Generally speaking, the coloration treatment may be carried out with thebath at any temperature between normal atmospheric temperature (say 70F.) and the boiling point of the solution.

hand, with temperatures in excess of about 220 F, the coloration takesplace so rapidly that it is difficult to control and, moreover, thecolor film which is formed is not particularly satisfactory orapparently integral with the surface of the steel in that it may bereadily rubbed off. The temperatures which I prefer to employ arebetween about 180 F. and about 210 F. In this range the best color filmsare produced, the time required is comparatively short and thereforecommercially practicable, and the color changes are not sufiicientlyrapid to prevent proper con-' trol. The steel should be removed from thesolution when it has attained a black coloration. The time requireddepends on the-acid concentration, the quantity of inhibitor, thetemperature of the solution and the analysis of the steel, and istherefore somewhat indefinite, but with the preferred solutions used atthe preferred temperatures the process usually takes from 15 to 20minutes. If the treatment is appreciably prolonged beyond the time whenthe steel acquires a black color the steel comes into what I term anoverdone'condition. That is to say it loses its intense black color andbecomes brown or iridescent. After coloring, the steel may be simplywashed and dried.

If it is desired, to produce a black color in the etched zones insteadof the zones or field about them the steel is merely subjected to thecoloration treatment for a period of time sufficient to overcome thepassivity imparted to the steel by the etching solution and thus effecta coloration thereof. Under my preferred procedure this may require aslong. as from 30 to 45 minutes. This However, with the lowertemperatures, I the time required to produce a satisfactory color 0 mayrun into hours or even days. On the other prolonged treatment necessaryto effect the col-' oration of the etched areas, of course. builds up acolor filmin the surface of the unetched portion. Due to the prolongedtreatment, however, this oxide film on the unetched zones is notparticularly adherent nor is the color a good black color due to theprolonged treatment. Usually the prolonged treatment will produce in theunetched zones either a brown discoloration or a non-uniform iridescentcoloration which indicates what I term an overdone condition.

Since the black color in the etched areas is somewhat below the surfaceof the surrounding zones or areas due to the depth of the etch, thecoloration in the zones surrounding the etched areas maybe quicklyremoved by simply passing the specimen under an abrasive sanding beltthus restoring the natural color of the steel to these zones and leavingthe black contrasting coloration only in the etched zones.

The color produced by my treatment is exceptionally fast and is notadversely affected by high temperatures or corrosive media which do notaffect the steel itself. It is substantially integral with the surfaceof the steel, andthe fact that it is in rather than merely upon thesurface of the steel makes it far superior to any baked-on lacquers orfills. 7

What I claim is:

1. Those steps in the method of producing a design upon the surface ofstainless steel which comprises passivating predetermined zones of saidsurface by subjecting said zones to the action of an etching medium, andthereafter subjecting said steel to the action of a solution containingsulphuric acid, water and an etching-inhibitingoxidizing agent until theunpassivated portions of said steel have acquired a dark coloration; thesulphuric acid constituting about 25% to about 65%, by weight, of theacid-water solution, and the inhibitor constituting from about 3% toabout 30%, by weight, of the whole solution-and being present in aquantity at least suflicient to prevent said solution from etching saidsteel when immersed therein.

2. The method of producing a design upon the surface of stainless steelwhich comprises passivating predetermined zones of said surface bysubjecting said zones to the action of an etching medium, coloring thesurface of said steel including said passivated zones by subjecting saidsteel to the action ofa solution containing sulphuric acid, water and anetching-inhibitingoxidizing agent, and thereafter removing the colorfrom the unpassivated portions of said surface by abrasion; thesulphuric acid constituting from about 25% to about 65%, by weight, ofthe acid-water solution, and the inhibitor consti-' tuting from about 3%to about 30%, by weight, of the whole solution and being present in aquantity at least suflicient to prevent said solution from etching saidsteel therein.

3. The method of producing a design on the surface of stainless steelwhich comprises coating predetermined zones of said surface with anetching resist, exposing the uncoated zones thereof to an etching mediumuntil said zones are etched to a depth slightly below the level of thecoated zones, removing the coating, subjecting those zones formerlycovered thereby to an abrasive treatment, and thereafter subjecting whenimmersed said steel to the action of a solution containing ficient toprevent said solution from etching said steel when immersed therein.

4. The method set forth in claim 1 in which the coloration treatment isconducted with the solution at a temperature between about F. and 220 F.

.5. The method set forth in claim 2 in which the coloration treatment isconducted with the solution at a temperature between about 180 F. and220 F.

6. The method set forth in claim 3 in which the coloration treatment isconducted with the solution at a temperature between about 180 F. and220 F.

7. The method of producing a design upon the surface of stainless steelwhich comprises passivating predetermined zones of said surface bysubjecting said zones to the action of an etching medium, and thereaftersubjecting said steel to the action of a solution containing sulphuricacid, water and an etching-inhibiting-oxidizing agent until thepassivated portions of said steel having acquired a dark coloration; theacid constituting from about 40% to about. 55%, by weight, of theacid-water solution, and the etching-inhibiting-oxidizing agentconstituting from about 8% to about 15%, by weight, of the wholesolution.

8. The method of producing a design upon the surface of stainless steelwhich comprises coating predetermined zones of 'said surface with aninhibiting-oxidizing agent; the acid constituting from about 40% toabout 55%, by weight, of the acid-water solution, and theetching-inhibitingoxidizing agent constituting from about 8% to about15%, by weight, of the whole solution.

9. The method of producing a design upon the surface of stainless steelwhich comprises subjecting predetermined zones of said surface to theaction of iron-perchloride to effect a passivation and etching thereof,subjecting the zones of said surface about said first mentioned zones toan abrasive treatment, and thereafter coloring said abraded zones bytreating the steel in a solution containing sulphuric acid, water and anetching-inhibiting-oxidizing agent at a temperature between about 180 F.and 220 F.; the

sulphuric acid constituting from about 25% to about 65%, by weight, ofthe acid-water solution and the inhibitor constituting from about 3% toabout 30%, by weight, of the whole solution and being present in aquantity at least suflicient to subjecting said steel to the action of asolution containing sulphuric acid, water and anetchinginhibiting-oxidizing 'agent, and thereafter removing the coloriromthe unpassivated portions by abrasion; the sulphuric acidconstituting from about 40% to about 55%, by weight, 01 the acidwatersolution and the etchlng-inhiblting-oxidizing agent constituting fromabout 8% to about 15%, by weight, of the whole solution.

11. Those steps in the method of producing a design upon the surface ofstainless steel which comprise treating predetermined zones only of saidsurface with a passivating agent whereby said zones are rendered moreresistant to oxidation than the untreated zones of said surface, is

